The color in electrochrome windows or other systems is modified through the effects of potential difference. To obtain an electrochrome condition, a layer of a transition metal oxide is placed between two electric conducting layers An electrolyte layer is interposed between the electrochrome layer and one of the two conducting layers.
Most of the ion-conducting elements in the first column of the MENDELEEV periodic table are suitable electrolytes. Electrochrome systems have thus been described using sulfuric acid or any other strong acid placed in contact with a layer of tungsten oxide. The major inconvenience of protonic electrolyte conductors, however, is their acidic nature which can result in the rapid deterioration of the other layers in the system if proper precautions are not taken. The longevity of such systems is therefore extremely short.
This defect can be reduced through the use of less corrosive alkaline ion conductor materials; in particular, lithium electrolytes are preferred in view of the high diffusion speed of lithium ions, especially in tungsten trioxide, which provides rapid coloration and decoloration of the system. Moreover, to improve the contrast between the colored and decolored states of the system, U.S. Pat. No. 4,350,414 shows how a counter electrode symmetrical to the electrode formed by the layer of electrochrome materials can be used to insert ions during the decoloration phase and to release them during the coloration phase.
This raises the technical problem of finding a suitable material for the counter electrode, that is to say, perfectly compatible with the electrolyte used, which does not decompose under an electric current and above all which does not have any coloration--or is in a decolored state when the layer of electrochrome material is also in decoloration. This last condition is essential for the realization of light transmissive windows, which might be used, for example, as windows for buildings.